• Organic carbon is the major control of redox conditions

 - OC generated during photosynthesis
 - Decomposed during respiration

 
Photosynthesis

• Reaction that converts CO₂ to organic matter and O₂
 
 
 
 
 
 

 - Process driven by energy from the sun
 - All oxygen would be consumed if no photosynthesis
 - e.g. Evolution of photosynthetic algae
 

 - Nitrogen, Phosphorous and trace elements
 - Photosynthesis occurs until essential nutrient depleted
 - generally phosphate is limiting nutrient
 - occasionally nitrate is limiting

 
• Redfield ratio:
 

 - approximate composition of organic matter

C₁₀₆H₂₆₃O₁₁₀N₁₆P₁

• More complex reaction better represents photosynthesis:
 
 
 

• Reflects importance of P in C-O balance

 - C/P molar ratio is 106
 - N/P molar ratio is 16
 - O/P molar ratio is 138

• When oxygen present, OM decays in reverse of photosynthesis:
 
 
 

• By products include

 - nitrate
 - phosphate
 - CO₂ – decreases pH

• Note: most decay reactions microbially mediated

- e.g. apple in Alvin

• There are no free electrons, so there must be corresponding compound that is reduced

 - e.g. oxygen reduced to H₂O

 
• Other dissolved solutes can oxidize the organic matter once oxygen is used up.

 - each reactant occurs at lower pe levels
 - each reaction has a different terminal electron acceptor:

• In other words:

 - terminal electron acceptors are oxidizing agents (they become reduced).

• These are very important reactions:
 
(1) Nitrate reduction:
 

 - denitrification: final product is molecular nitrogen:
 

 - represents conversion of essential nutrient to inert molecule

 - reduction to Nitrite:
 

 - reduction to ammonia:

 
• Ammonia also released from decomposition of amino acids in proteins (part of organic matter)
 

• Ammonia can raise pH by generation of ammonium:
 

 
(1) Ferric iron (and Mn) reduction:
 
 
 
 
 

 - more common in ground water where metal oxides more concentrated.  Little in surface water

 - Fe²⁺ generally precipitates as carbonate or sulfide depending on solution chemistry

• The product is generally sulfide.

• Sulfate-reducing bacteria generally can use only small molecules > 20 C, e.g.

formate:
acetate:
lactate:
 
• Implications of these reactions:
 

 - sulfides commonly toxic
 - can be used by oxidizing bacteria for chemosynthesis
 - sediment color change as mineral change from oxides to sulfides
 - important for metal chemistry

* some adsorbed to surface are released
* others precipitate as sulfides

 - essentially the breakdown of complex carbohydrates to simpler molecules
 - products often used by sulfate reducing bacteria

e.g.:
 
 

 
• Each group of reactions requires specific bacteria
• Bacteria derive energy from reactions

 - essentially catalyze breakdown of unstable to stable system
 - reactions occur in approximate succession with depth in the sediment

Succession:
 
 

 
Redox Buffering

• pe can be buffered just like pH

 - depends on the electron receptor present
 - example of surface water

• With oxygen presents various reactions could control pe:
 
 
 
 

Each reaction keeps pe at particularly value until all reactant consumed
 
• When all oxygen consumed, sulfate reduction becomes important:

 - pe obtains value for sulfate reduction reaction:

• The pe of waters would be in one of the buffered ranges

 - can be determined on basis of presence/absence of oxygen and sulfate

 - buffering could include solid phases

• Example profiles

- Equatorial Atlantic: slow sedimentation, little organic matter
- Nearshore NC: fast sedimentation, high organic matter

 
Lakes

• In temperate climates, lakes are vertically stratified:

 - Epilimnion – warm, low density water, well-mixed from winds.
 - Metalimnion (thermocline) – rapid decrease in T with depth
 - Hypolimnion – uniformly cold water at base of lake.

• The stratification is stable: there is little mixing between Hypolimnion and epilimnion
 
• At end of summer, epilimnion reaches temperatures same as or lower than hypolimnion.

 - metalimnion breaks down
 - wind completely mixes water column
 - called Fall Turnover
 

• With warming temperatures in spring additional mixing occurs: Spring Turnover

• Dimictic: turn over twice a year
• Monomictic: turn over once a year
 
• Oxygen content (redox conditions) depend on turnover
 

 - Oxygen in hypolimnion decreases as organic matter falls from surface zone and is oxidized
 - The amount of oxygen used depends on production on photic zone
 - The production depends on nutrients, usually phosphate

 - Oligotrophic: low supply of nutrients, water oxygenated at all depths
 - Eutrophic: high supply of nutrients, hypolimnion can be anaerobic.

- Difficult to reverse process
- Nutrients (P) buried in sediments because adsorbed to Fe-oxides
- When buried Fe-oxides reduced and form Fe (II) and Fe-carbonates and sulfides
- Released P returns to lake

• The ocean continually turn over

 - Broecker’s “conveyer belt”
 - Nutrient distribution controlled by decay in water column and circulations
 - (Lakes: nutrients mostly from input water)

• Oxygen profiles controlled by settling organic matter

 
• Silled basins

 - Cariaco Basin – Venezuela
 - Sanich Inlet – B.C.
 - Santa Barbara Basin, California

• Little deep water circulation

 - oxygen rapidly depleted
 - may go to sulfate reduction in water column
 - Sediments affected

  * Black (sulfides)
  * Laminated (no bioturbation)

 
• Mechanisms controlling redox in sediments

 - sedimentation rate
 - organic matter content
 

 - controls depth of diffusion from oxygenated water
 - i.e. time in high pe water

• Difficult to generalize about controls on redox reactions

• Nonetheless, most important controls appear to be:

 (1) Oxygen content of recharge water

  * enter through fractures (sinkholes) - aerobic
  * percolate through soils (carbon rich) –
anaerobic

 - aquifers vary in amount of organic carbon
 - “quality” of carbon variable, usually refractory
 - refractory because a) old, b) subject to heat
 

(3) Distribution of redox buffers

 - aquifers may have large amounts of Mn and Fe oxides

(4) Circulation of ground water

 - flow rates, transit times, residence times
 - longer residence times generally mean lower pe